Global Trends in Day‐Night Compound Thermal Extremes in Lakes

Abstract Lake thermal extremes, both hot and cold, profoundly impact aquatic ecosystems, yet day‐night compound events remain understudied. Using the hourly resolution Global LAke Surface water Temperature (GLAST) data set (1981–2020), we found day‐night compound hot (cold) extreme events in lakes occurred annually at 6.9% (6.9%) frequency, lasted 3.7 (3.8) days, and had intensities of 2.3 (−2.1) °C. These metrics were ∼5 times more frequent, lasted longer, and were ∼1.5 times more intense than daytime‐only or nighttime‐only extremes. Over four decades, compound hot events increased in frequency (+1.8% decade −1 ), duration (+0.56 days decade −1 ), and intensity (+0.14°C decade −1 ), while compound cold events declined, driven by rising mean lake temperatures ( R 2 > 0.76). Lagging air temperature extremes by 1.7 days, lake temperature extremes were less intense but more frequent and persistent due to thermal inertia. These findings highlight the need for adaptive strategies to mitigate the escalating threats posed by persistent day‐night extreme thermal conditions on aquatic habitats.